1. Field of the Invention
The present invention relates to a digital image forming equipment in which image is formed by reflecting and scanning laser light emitted from a source of light onto a photosensitive body by means of a polygon mirror driven by a drive motor (so-called a polygon motor).
2. Description of the Prior Art
In a digital copying machine which is one of the examples of this kind of a digital image forming equipment, laser light is irradiated on the surface of a photosensitive drum on the basis of information of copy images which have been digitally signalized, thereby causing electrostatic latent images to be formed. At this time, the above laser light which is emitted from the source of light is reflected and scanned onto the surface of a photosensitive drum by means of a polygon mirror rotated by a polygon motor.
The above polygon motor usually rotates at such a high speed as 5,000 to 20,000 r.p.m. and is requested to rotate at a stabilized condition since high precision is required for scanning the laser light. As it usually takes about 5 seconds from start of the rotation of the above polygon motor to reaching the stabilized rotation condition, conventionally the polygon motor starts to rotate as soon as the power source of a copying machine is turned on, and it is so controlled that it can usually consecutively rotate while waiting unless the power source is turned off. Quick response for copying jobs by an operator have been thus secured.
As described in the above, as the polygon motor consecutively rotates at a high speed for a longer period of time, for instance, a very high grade of bearing such as a non-contact bearing of air gap type is required. There are some problems that a non-contact type bearing as shown in the above becomes so complicated in structure and the price thereof may become comparatively high.
Furthermore, as the above polygon motor rotates at a high speed, rotary noise whose frequency is comparatively high is emitted in accompanying with the rotation of the polygon motor. In addition, the above rotary noise is continuously emitted for a longer period of time since the above polygon motor consecutively rotates, thereby causing another problem to occur. That is, the polygon motor gives a sense of uncomfortable feeling to every person who is in the vicinity of the copying machine.